1. Field of the Invention
The present invention relates to a technology for conveying a recording medium to an image transfer unit in an electrophotographic image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as a copier and a printer, a recording medium is conveyed by a registration roller unit to an image transfer unit at the right time, as described in, for example, Japanese Patent No. 2893540 and Japanese Patent Application Laid-open No. 2002-265097. Specifically, the technology for conveying the recording medium is designed to accurately transfer an image carried on an image carrier, such as a photosensitive drum and an intermediate transfer belt, to the right position on the recording medium without any misalignment of the image. The term “misalignment” mentioned here indicates “deviation” from a predetermined position to which the image is supposed to be transferred.
More specifically, Japanese Patent No. 2893540 discloses a technology for butting a recording medium against a butting member (gate device) to position a leading edge of the recording medium, holding the recording medium by a registration roller unit provided on the downstream side of the butting member, moving the registration roller unit in a width direction of the recording medium in the state of holding the recording medium thereby, correcting the misalignment of the recording medium in its width direction (direction perpendicular to a conveying direction), and then conveying the recording medium to the image transfer unit.
Japanese Patent Application Laid-open No. 2002-265097 discloses a technology for butting a recording medium against a butting member (butting surface) to correct a skew of the recording medium, and then conveying the recording medium to the image transfer unit by a registration roller unit provided on the upstream side of the butting member.
However, in the conventional technologies, there may be a case where the misalignment of the recording medium is not fully corrected and an image is not thereby accurately transferred to the right position on the recording medium.
Specifically, in the technologies disclosed in Japanese Patent No. 2893540 and Japanese Patent Application Laid-open No. 2002-265097, the recording medium is held by the roller unit provided on the upstream side of the butting member after the recording medium is butted against the butting member. At this time, the leading edge of the recording medium butted against the butting member is displaced caused by a difference in rigidity of recording media or the like, and the position in the conveying direction (hereinafter, “longitudinal registration” as necessary) thereby deviates, to cause a deviation of a transfer start position of an image with respect to the recording medium conveyed to the image transfer unit.
Moreover, in the technology disclosed in Japanese Patent No. 2893540, even when the deviation of a position of the recording medium in its width direction (hereinafter, “lateral registration” as necessary) is corrected in a state of holding the recording medium by the registration roller unit provided on the downstream side of the butting member, another deviation may occur in the longitudinal registration of the recording medium.
Particularly, these problems cannot be ignored because deviations or skews of the longitudinal registration and the lateral registration of the recording medium may easily occur in a high-speed image forming apparatus in which the recording medium is conveyed at a high speed.
Furthermore, in the conventional technologies, there are some cases where the skew of the recording medium is not fully corrected and thus the image is not appropriately transferred to the right position on the recording medium.
More specifically, there is a case where the butting member is not accurately assembled in the conveying device and the butting member is thereby tilted in the width direction with respect to the conveying direction of the recording medium. In this case, if the entire leading edge of the recording medium butts against the tilted butting member, then the butting results in occurrence of a skew, corresponding to the tilt of the butting member, in the recording medium of which skew should be corrected. Such a problem as above becomes particularly significant when the butting member is unitized and is, therefore, detachably attached to the conveying device.
Furthermore, when specific skew occurs depending on a type of the recording medium and the use environment of the device, even if the skew of the recording medium is corrected at the position of the butting member, the recording medium is again skewed during conveyance of the recording medium from the butting member to the image transfer unit, and thus the image cannot be formed at the right position on the recording medium.
Particularly, such a problem cannot be ignored because the skew of the recording medium easily occurs in a high-speed image forming apparatus in which the recording medium is conveyed at a high speed.